The invention relates to a transmission system comprising a transmitter and a receiver coupled together by way of a channel, the receiver including a circuit arrangement for automatic gain control. The circuit arrangement includes a differential amplifier having an input for receiving a signal to be controlled, an output for presenting a controlled output signal and a control input for adjusting the gain as a function of a control signal; and a generator for producing the control signal by rectifying the output signal produced by the differential amplifier. The invention likewise relates to a receiver and a circuit arrangement for automatic gain control.
When serial data is optically transmitted, a photodiode arranged on the receiving side of a light waveguide converts light pulses obtained from a transmitter into an electric data signal. An AC-coupled preamplifier amplifies the low voltage level of the (converted, i.e., electrical) data signal to produce a logic voltage level necessary for driving integrated logic circuits. As a result of the AC coupling of the preamplifier, however, the DC component in the data signal is lost. The lost DC component may be recovered in a circuit having quantized feedback. Prior-art amplifier circuits having quantized feedback (IEEE Transactions on Magnetics, Vol. MAG. 14, No. 4, July 1978, pp. 218 to 222; NTZ, Vol. 30 (1977), No. 9, pp. 712 to 717), however, need to have at their input a data signal with a constant voltage level. Therefore, the quantized feedback circuit is to be preceded by a circuit arrangement for automatic gain control.
DE 25 00 654 B1 has already disclosed a circuit arrangement for level control of a data signal. This data signal forms part of a compound signal which is transmitted together with a pilot signal over a transmission link. Signals signalling the actual level and the nominal level of the pilot signal are then compared in a comparator, and, as a result, a control voltage is derived which is applied to a control stage controlling the level of the data signal. This prior-art circuit arrangement is unsuitable for maintaining the voltage level of data signals that do not have such a pilot signal at a constant level.